Optical extinction monitor using cw cavity enhanced detection

Abstract

We present details of an apparatus capable of measuring optical extinction (i.e., scattering and/or absorption) with high precision and sensitivity. The apparatus employs one variant of cavity enhanced detection, specifically cavity attenuated phase shift spectroscopy, using a near-confocal arrangement of two high reflectivity (R approximately 0.9999) mirrors in tandem with an enclosed cell 26 cm in length, a light emitting diode (LED), and a vacuum photodiode detector. The square wave modulated light from the LED passes through the absorption cell and is detected as a distorted wave form which is characterized by a phase shift with respect to the initial modulation. The amount of that phase shift is a function of fixed instrument properties-cell length, mirror reflectivity, and modulation frequency-and of the presence of a scatterer or absorber (air, particles, trace gases, etc.) within the cell. The specific implementation reported here employs a blue LED; the wavelength and spectral bandpass of the measurement are defined by the use of an interference filter centered at 440 nm with a 20 nm wide bandpass. The monitor is enclosed within a standard 19 in. rack-mounted instrumentation box, weighs 10 kg, and uses 70 W of electrical power including a vacuum pump. Measurements of the phase shift induced by Rayleigh scattering from several gases (which range in extinction coefficient from 0.4-32 Mm(-1)) exhibit a highly linear dependence (r(2)=0.999 97) when plotted as the co-tangent of the phase shift versus the expected extinction. Using heterodyne demodulation techniques, we demonstrate a detection limit of 0.04 Mm(-1) (4 x 10(-10) cm(-1)) (2sigma) in 10 s integration time and a base line drift of less than +/-0.1 Mm(-1) over a 24 h period. Detection limits decrease as the square root of integration time out to approximately 150 s.

Airborne and laboratory studies of an IAGOS instrumentation package containing a modified CAPS particle extinction monitor

“…Considering 1 second averaged data, the standard deviation is 0.274 Mm ¡1 and decreases to 0.046 Mm ¡1 for 60 seconds running averages. The Allan analysis is in accordance to what has been shown by Massoli et al (2010) and Kebabian et al (2007) for CAPS systems operating at ambient pressure condition.…”

“…Effects of changes in pressure and temperature on the gas Rayleigh scattering coefficient in the cell compared to the baseline period are taken into account by a correction algorithm based on the ideal gas law. The residence time of the sample inside the measurement cell ensures the equilibrium of sample and cell temperatures (Kebabian et al 2007). The CAPS PM ex system used in this study operates at a wavelength of 630 nm and thus shows minimal interference with absorbing gaseous species such as nitrogen dioxide; a NO 2 volume mixing ratio of 1 ppb produces an extinction coefficient of 0.04 Mm ¡1 at the operation wavelength.…”

“…An alternative technology, the Cavity Attenuated Phase Shift Particle Extinction Monitor (CAPS PM ex ), was commercially introduced in 2007 to directly measure the extinction of particles (Kebabian et al 2007;Massoli et al 2010). The instrument is available at several operation wavelengths in the visible range.…”

“…In contrast to the deployment of a laser in the CDR, it uses light emitting diodes (LED) as a light source. The CAPS PMex can be readily applied in mobile sampling systems since it is compact and robust (Kebabian et al 2007), with a relatively low-cost set up. The CAPS PM ex operational principle has been successfully assessed in optical closure studies and deployed at ground pressure conditions with artificially generated and ambient aerosols (Massoli et al 2010;Petzold et al 2013b) and high concentration exhaust aerosols (Yu et al 2011).…”

Organic aerosol composition and sources in Pasadena, California, during the 2010 CalNex campaign

“…Volume concentrations of the different PALMS particle classes were calculated by multiplying the volume concentration size distribution determined from the SMPS or WLOPC measurements by the fractions of the different PALMS particle types in each size bin. Particle optical extinction for PM 1 was measured at 532 nm and 630 nm by a Cavity Attenuated Phase Shift instrument [Kebabian et al, 2007;Massoli et al, 2010]. All of the online measurements described in this paragraph were located at the CalNex ground site described above.…”